The present invention relates to the field of fiber reinforced materials with matrix metal being metal, and more particularly relates to such a fiber reinforced material in which the reinforcing fiber material is carbon fibers and the matrix material is a magnesium alloy.
In the field of composite materials the use of carbon fibers, either long or short, as reinforcing fibers for a matrix metal is considered to have promise as a combination, because the carbon fibers have high strength and high rigidity, while the appropriate matrix metal can have good flexibility and cracking resistance and so on; however, various problems can occur. As a matrix metal various types, especially of light metals, have been considered, and particularly aluminum and its alloys are suitable, and further magnesium and its alloys have many attractive features. In the case of a matrix which is a light metal which is not itself aluminum or is an alloy of a light metal which is not itself aluminum, such as magnesium or magnesium alloy for example, then it is common to add a certain amount of aluminum or zirconium to the light metal or alloy thereof in order to improve its properties, for example in order to ensure finer crystallization thereof and in order to improve the mechanical and thermal properties thereof; however, this added aluminum or zirconium should be restricted to be not more than a certain amount, and it is preferred to utilize aluminum rather than zirconium, on account of the relatively high price of zirconium. Now, it is a known problem that, in the process of manufacturing a composite material by compositing such a light metal or metal alloy containing aluminum in the molten state with carbon fibers, carbide producing reactions are liable to occur at the surface of the carbon fibers, by the carbon therein reacting with the aluminum content of the matrix metal, and this can in some cases severely impair the effectiveness of the carbon fibers as reinforcing material. In fact, to consider the various classes and types of carbon fibers, this impairment by carbidization of the carbon fibers is relatively light and unimportant in the case of so called high elasticity type carbon fibers which have relatively high graphitization level, but is relatively great and even is extreme in the case of so called high strength type carbon fibers which have relatively low graphitization level, so much as that these so called high strength type carbon fibers which have relatively low graphitization level are thereby made brittle and lose their strength, so as to be of almost no practical use as reinforcing fiber material for the composite material.
Now there is a per se known method of limiting this deterioration of the carbon fibers by carbidization, which is described in Japanese Patent Publication Ser. No. Sho. 49-18891 (1974): in this method, a relatively large amount of an element such as titanium or zirconium, which has a stronger tendency to form a carbide by combination with the carbon fibers than does the aluminum included in the matrix metal, is added to the matrix metal. Thus, when the carbon fibers are composited with the molten matrix metal, a layer of relatively innocuous carbide such as titanium carbide or zirconium carbide (which are more stable than aluminum carbide) is positively formed on the surfaces of the carbon fibers, so that the carbidization reaction between said carbon fibers and the aluminum contained in the matrix metal is limited. A disadvantage of this method, however, is that not only can the above described reaction not be satisfactorily restricted and controlled, but there is also the problem that the formation of a layer of brittle carbide on the surfaces of the carbon fibers causes a reduction in the strength of the resultant carbon fiber reinforced composite material, presumably because the stress propagation qualities between the carbon fibers and the matrix metal at the surfaces of the carbon fibers are impaired. Further, since such metals as titanium or zirconium are required to be used as additive metals, the cost of the process is high.
Another per se known method of limiting this deterioration of the carbon fibers by carbidization is performed by, before compositing the carbon fibers with the matrix metal containing aluminum, first forming a layer of carbide such as titanium carbide or zirconium carbide on the surfaces of the carbon fibers in a separate step. In this case, the carbide formation reaction can be satisfactorily restricted and controlled, and the layer of such carbide can be ensured to be more perfect, but a special step is required for the formation of this titanium carbide or zirconium carbide layer, which increases cost and production complexity. Further, the problem of reduction in the strength of the resultant carbon fiber reinforced composite material caused by the formation of the brittle carbide layer on the surfaces of the carbon fibers is not resolved, and this carbide layer inevitably reduces the intimacy of the contact and adhesion between the carbon fibers and the matrix metal, and thereby it becomes impossible to adequately improve the strength of the resulting composite material.